Cemented carbide is used for many metal cutting applications at the present time. It is usually a sintered product resulting from a mixture of powdered tungsten carbide and a binder metal, usually cobalt. The addition of titanium carbide, hafnium carbide, and tantalum carbide or niobium carbide, or other metal carbides, in small percentages improves the resistance of cemented carbide to cratering but may cause a decrease in tool strength. Improved abrasive wear resistance and also resistance to cratering without a decrease in tool strength has been sought through the expedient of applying a thin coating on the surface of the cemented carbide. This makes it possible to achieve maximum resistance to abrasive wear and cratering while the substrate has suitable resistance to breakage and deformation. One of the first coatings utilized was titanium carbide which not only improved the tool life considerably but also permitted a substantial increase in cutting speeds. Another coating to achieve commercial recognition has been a so-called ceramic coating in the form of an oxide, such as aluminum oxide, Al.sub.2 O.sub.3.
Chemical vapor deposition of the metal carbides on cemented tungsten carbide substrates has been the subject of investigation and use in the last two decades as evidenced by U.S. patents issued in 1960 to Ruppert, U.S. Pat. No. 2,962,388 and 2,962,399, and also a U.S. patent to Glaski, U.S. Pat. No. 3,640,689, issued Feb. 8, 1972. Thin coatings of nitrides, silicides, and carbides of the metals in Groups IVB, or VB and VIB of the Periodic Table have been applied to cemented carbide substrates for improving the wear characteristics of cutting inserts.
A further development, as indicated above, has been the addition of a surface layer of a refractory oxide such as Al.sub.2 O.sub.3 or zirconium oxide as described in U.S. patents to Hale, U.S. Pat. No. 3,736,107, Lux, U.S. Pat. No. 3,836,392 and Lindstrom, U.S. Pat. No. 3,837,896.
While these layers, when added to the hard substrate, have improved the wear characteristics of cutting inserts, there have been continuing problems with respect to abrasive wear of the edge or flank of the metal cutting cemented carbide tools as well as cratering of the rake face of the tools.
It is accordingly an object of the present invention to provide a coated carbide insert which has an improved resistance to abrasion at the edge or flank of the insert while also achieving an improved resistance to cratering on the rake face of the tool.
It is a further object to provide a coated insert which has greater lubricity, a less reactive surface with materials being machined, a high hardness, and a better thermal barrier to protect the substrate.
An additional object of the invention is the provision of a process for producing an adherant, dense multilayer coating of the desired different compositions in each layer on the cemented carbide substrate.
Briefly, it has been discovered that the application of a coating of metallic nitrides or carbides, especially nitrides and carbonitrides, on the refractory oxides gives improved performance over the refractory oxides despite the fact the substances themselves on a substrate do not produce such superior results.
Other objects and features of the invention will be found in the following description and claims in which the invention is described together with details directed to those skilled in the pertinent arts of the manner and process of making and using the invention, all in connection with the best mode presently contemplated for the practice of the invention.